Photonic crystal optical fibres, also called “microstructured fibres” or “photonic forbidden band fibres”, use a light confinement mechanism based on the periodicity of their index structure. Two types of microstructured fibres may principally be distinguished:                high index core fibres, in which a cladding including a periodic arrangement of low index inclusions surrounds a defect, that is to say an absence of inclusion, which serves as core,        low index core fibres, in which a cladding including a periodic arrangement of high index inclusions surrounds a defect, that is to say an absence of inclusion, which serves as core.        
In the first case of high index core fibres, it is possible to assimilate the index of the microstructured cladding with an average index that is less than the index of the core. The confinement mechanism may thus be assimilated with the guiding mechanism in the dielectric waveguide.
In the second case of low index core fibres, the periodic index structure of the microstructured cladding leads to the formation of forbidden energy bands: thus, certain wavelengths cannot propagate in the microstructured cladding. Consequently, if a defect is introduced into the structure that can support a mode of which the effective index is lower than that of the core and is located in the forbidden band, light will be confined in this defect. In other words, the photonic crystal microstructure of the cladding will behave as a reflective mirror for the photons of which the directions of propagation are transversal with respect to the axis of the fibre and of which the wavelength lies within the forbidden band.
Several types of microstructured fibres exist:                In the case of a microstructured fibre of Bragg fibre type, the photonic crystal is obtained by the alternation of concentric layers of quarter wave type of two materials of different indices, for example polymethyl methacrylate PMMA on the one hand and polystyrene on the other hand.        In the case of a microstructured fibre of “Holey fibre” type, tubular holes parallel to the axis of the fibre, filled with air, that is to say of optical index equal to 1, surround the core of the fibre, whereas the remainder of the matrix of the fibre is formed of a material, for example polymer, of optical index greater than 1. The holes produce a forbidden band and may be arranged according to a hexagonal “honeycomb” network, according to a square network or according to one or more concentric rings around the core.        
Furthermore, fluorescent concentrators are notably developed in the field of solar energy research in order to improve the efficiency of solar cells. A fluorescent concentrator is typically made of PMMA and contains a fluorescent dye. The fluorescent dye has an absorption spectral band and an emission spectral band. When a photon, of wavelength λa lying within the absorption spectral band of the dye, reaches the concentrator and encounters a molecule of dye, it is absorbed by said molecule then re-emitted at a wavelength lying within the emission spectral band of the dye. The emission wavelength λe is greater than the absorption wavelength λa, which reflects the fact that the re-emitted photon has lower energy than the photon initially absorbed.
The field of optical devices with photon flipping is based on the two fields which have been mentioned, that is to say photonic crystal optical fibres and fluorescent concentrators. An optical device with photon flipping thus typically comprises a core area, a flip area including a fluorescent flip dye and a microstructured cladding area.
The microstructured cladding area has an allowed spectral band and a forbidden spectral band. Photons of wavelength lying within the allowed spectral band can propagate in the cladding area, whereas photons of wavelength lying within the forbidden spectral band cannot propagate in the cladding area.
The fluorescent dye is chosen so that its absorption spectral band overlaps at least partially the allowed spectral band of the microstructured cladding area, and so that its emission spectral band overlaps at least partially the forbidden spectral band of the microstructured cladding area.
An optical device with photon flipping thus makes it possible to convert an incident light flux, captured laterally by the microstructured cladding area, into a quasi-monochromatic and anisotropic beam propagating in the core area. Nevertheless, optical devices with photon flipping of the prior art only have low conversion efficiency, less than 10% for thicknesses of light guide greater than a millimetre.